The IP multimedia subsystem is an architectural framework for providing packet data to mobile devices in a standardized fashion. Such IMS network allows for voice calls over a packet system rather the circuit switched system. It allows for other functionalities such as push to talk (PTT), and in particular, mission critical push to talk (MCPTT) used for first responders.
IMS authentication authenticates both a private identifier and a public identifier for use on the IMS network. However, the IMS authentication does not allow different individual IMS public user identities to be separately authenticated with the same or different IMS private user identities. This can lead to problems in various situations. For example, if used by first responders, a MCPTT application may need a device to be authenticated by a different public identity than that tied to the private identity of the device.